1. Field of the Invention
The present invention generally relates to the sensing of magnetic fields, and more specifically to a magnetoresistive sensor suitable for fabrication using integrated circuit technology which produces a high output voltage and is insensitive to temperature variation.
2. Description of the Related Art
Magnetoresistive sensors are widely used for sensing magnetic field flux, and more specifically translational and rotational speed and position as analogous variations of magnetic flux. A basic magnetoresistive sensor 10 of known design is illustrated in FIG. 1, and includes a magnetoresistor 12 which is connected in series with a fixed resistor 14. A source voltage V+ is applied across the resistors 12 and 14, whereas the junction of the resistors 12 and 14 is connected to the non-inverting input of an operational amplifier 16. A reference voltage VR1 is applied to the inverting input of the amplifier 16.
A toothed wheel 20 made of a ferromagnetic material such as steel is mounted for integral rotation on a member (not shown) whose rotational speed or position is to be sensed. The wheel 20 may be mounted on a crankshaft of an automotive vehicle, for example, for measuring engine speed (RPM), or on a wheel shaft for measuring vehicle speed.
The magnetoresistor 12 is disposed between the wheel 20 and a magnet 22 such that a magnetic field is developed between the wheel 20 and magnet 22, with the magnetic field flux passing through the magnetoresistor 12. The resistance of the magnetoresistor 12 increases as the magnitude of the applied magnetic flux increases. The flux is maximum when one of the teeth of the wheel 20 is aligned with the magnet 22, and minimum when one of the gaps between the teeth is aligned with the magnet 22.
As the magnetic flux and thereby the resistance of the magnetoresistor 12 increase, the voltage drop across the magnetoresistor 12 increases and the voltage at the junction of the resistors 12 and 14 and thereby at the noninverting input of the amplifier 16 decreases, and vice-versa. The reference voltage VR1 is selected to be equidistant between the minimum and maximum voltages appearing at the junction of the resistors 12 and 14. The amplifier 16 produces an output voltage Vout.
As each tooth of the wheel 20 passes through alignment with the magnet 22, the voltage at the non-inverting input of the amplifier 16 crosses below the voltage at the inverting input, and the amplifier 16 produces a negative output voltage Vout. As each gap between teeth of the wheel 20 passes through alignment with the magnet 22, the voltage at the non-inverting input of the amplifier 16 crosses above the voltage at the inverting input, and the amplifier 16 produces a positive output voltage Vout. In this manner, the sensor 10 produces a periodic output signal Vout at a frequency which is proportional to the rotational speed of the wheel 20.
FIG. 2 illustrates a sensor 30 which is similar to the sensor 10, but further includes a voltage divider consisting of resistors 32 and 34 which produces the reference voltage VR1.
The fixed resistor 14 constitutes an output load for the magnetoresistor 12. As the resistance of the magnetoresistor 12 increases, the voltage thereacross increases as described above. However, the current flow through the resistors 12 and 14 decreases by an amount corresponding to the increased resistance of the magnetoresistor 12. Thus, the change in voltage across the magnetoresistor 12 is less than that which would be produced if the current through the resistors 12 and 14 were maintained constant.
This limits the output voltage of the sensor 10 to substantially less than a maximum possible value. In the impedance matched case where the resistances of the resistors 12 and 14 are equal, the output voltage swing is one-half that which would be produced if the current through the resistors 12 and 14 did not vary with the resistance of the magnetoresistor 12.
In addition to the limited output swing discussed above, a typical magnetoresistor 12, that is formed of indium antimonide or similar material is very sensitive to temperature variations. A high performance magnetoresistive sensor is typically required to operate over a temperature range of -40.degree. C. to +250.degree. C. The resistance of the magnetoresistor 12 decreases as the temperature increases, with the resistance at the high end of the temperature range being on the order of one-half the resistance at the low end. This variation of resistance in accordance with temperature produces a corresponding variation in output voltage, which is a major source of error.
U.S. Pat. No. 5,038,130, entitled "SYSTEM FOR SENSING CHANGES IN A MAGNETIC FIELD", issued Aug. 6, 1991, to R. Eck (one of the present inventors) et al, discloses the fabrication of resistors 12 and 14 from the same magnetoresistive material and having the same geometry, using integrated circuit technology. Hall effect shorting strips or other structures are formed on the magnetoresistor 12 such that it has a much higher magnetoresistance than the resistor 14.
The resistances of the resistors 12 and 14 vary in accordance with temperature in the same manner, such that the voltages across the resistors 12 and 14 and thereby the voltage at the non-inverting input of the amplifier 16 do not vary with temperature. However, the magnetoresistor 12 is still resistively loaded by the fixed resistor 14, and the output voltage is still undesirably limited thereby.
U.S. Pat. No. 4,727,323, entitled "MAGNETO-RESISTIVE DIFFERENTIAL SENSOR SYSTEM", issued Feb. 23, 1988, to E. Zabler discloses a sensor which is illustrated in present FIG. 3 and designated as 40. The sensor 40 includes magnetoresistors 42 and 44 connected in series between points 46 and 48, with the junction of the magnetoresistors 42 and 44 grounded.
Fixed resistors 50 and 52 are also connected in series across the points 46 and 48, with the junction of the resistors 50 and 52 connected to the inverting input of an operational amplifier 54. A reference voltage VR2 is applied to non-inverting input of the amplifier 54, with the output of the amplifier 54 applied to control a dual current source 56. The output voltage Vout of the sensor 40 is taken across the points 46 and 48.
The current source 56 sources a current I1 into the point 46 and sinks an equal current I1 out of the point 48. The current I1 is maintained constant at a given value of temperature, but is controlled by the amplifier 54 to vary in accordance with temperature. The resistances of the magnetoresistors 42 and 44, and thereby the voltage at the inverting input of the amplifier 54 vary with temperature.
The amplifier 54 compares the voltages at its two inputs and controls the source 56 in accordance with the difference therebetween to increase the current I1 as the resistances of the magnetoresistors 42 and 44 increase, and vice-versa, to maintain the voltages across the magnetoresistors 42 and 44 and thereby the output voltage Vout constant with temperature.
Although attaining temperature compensation, the arrangement of FIG. 3 requires a complicated dual current source and feedback loop to vary the current I1 as a function of temperature, and is difficult to implement in an integrated circuit.